In recent years electrophysiological studies with intact nerve have provided considerable information regarding the functional properties of the sodium channel particularly with respect to ion selectivity and gating properties. On the other hand, the chemical and structural bases of many of these functions still remains obscure. We propose to examine the structural and functional characteristics of the Na+ channel in excitable membranes by measurement of the distances between functional sites using fluorescence resonance energy transfer. Characterization of important functional sites with neurotoxins and fluorescent analogs by equilibrium binding methods are described in order to establish the mode of interaction of the ligands with the macromolecule. Placement of spectroscopic probes at functional sites on the Na+ channel in rat axolemma and sarcolemmal membranes, and lobster nerve membranes and using fluorescence energy transfer and dipolar relaxation will enable use to directly measure the conformational states accessible to the channel for ion selection and gating. A static and dynamic structural mapy of the important sites on the Na+ channel will be obtained from this information. Kinetic studies using conventional and fast reaction techiques will be undertaken to deduce possible mechanisms for the flux of cations through the channel. In particular, experiments are described where the dynamics of substrates Na+ and Tl+ and modulatory ligands (e.g. polypeptide toxins) interacting with the ion channel will be examined. The kinetics of cation flux will be done by pulsed-quench flow and stopped-fluorescence to elucidate the elementary steps of cation flux and thermodynamic interactions with the channel. Microscopic visualization and counting of Na+ channels in myelinated, premyelinated, amyelinated, and demyelinated fibers will be done by fluorescence microscopy and by high-resolution electron microscope autoradiography. The study of the Na+ channel is of particular importance due to its key role in the propagation of the nerve impulse and since a number of neurological disorders are directly associated with its impaired or altered activity.